This invention relates to a tire having an airtight layer with a ductile layer.
Tubeless tires have an interior surface of low air permeability in order to avoid deflation of the tire and to protect the sensitive internal areas of the latter against intake of oxygen and water, such as the plies containing wire cords sensitive to oxidation. Today such protection of the interior surface of tires is provided by coatings made of butyl rubber.
Since fuel economy and the need to protect the environment have become a priority, it is desirable to produce airtight coatings with a weight and a hysteresis as low as possible. Performances in terms of airtightness of butyl rubber are linked to a not negligible minimum thickness of rubber (in the order of one millimeter) and therefore to a certain weight, which makes it impossible to respond to these new requirements efficiently.
In order to accomplish such objective of weight reduction of the coating, while maintaining good airtightness properties, numerous solutions have been proposed. The very great majority of those solutions have concentrated essentially on the use of materials of very low air permeability other than butyl rubber. In particular, the use of lamellar composite structures has been proposed, consisting of one or more layers of barrier material, to which is added or not an adhesive layer ensuring maintenance on the compound to be coated.
By way of illustrative examples of this prior art, U.S. Pat Nos. 4,874,670 and 5,036,113, describe the development of an elastomer inner liner consisting of a photoreticulated polymer film maintained in the internal compound of the casing by means of an adhesive (formophenol resin or alcohol acrylate with polymerized long chain). In spite of a reduced thickness and air permeability relative to the butyl rubber-base inner liner, this film has a mediocre adherence temperature resistance as well as overly high vitreous transition temperature values (Tg) for use as a tire casing inner liner.
Furthermore, the oxygen permeability of polyurethanes increases in the presence of water, which is unfavorable for the role of protection by the inner liner of the internal plies against corrosion of the cords, as well as for the maintenance of inflation pressure. The same problem appears in U.S. Pat. No. 5,264,524, where the barrier coating consists of an acrylonitrile/butadiene/polyurea copolymer.
Other solutions have also been proposed, such as presented in U.S. Pat. No. 5,236,030, which describes the lining of a tire casing consisting of a thin polyethylene film of ultrahigh molecular weight (UHMWPE), the airtightness and lightness of which are augmented in relation to a butyl rubber inner liner, without requiring the use of an adhesive layer, although no proof of adhesive sufficiency of the UHMWPE layer is presented. This layer is placed directly inside the tire casing, which probably results, upon forming in the tire casing and upon vulcanization, in an alteration of crystallinity of the film, which is a parameter essential for low air permeability.
In the variant applications presented in patents JP 4062009 and JP 4212602, the UHMWPE film in emulsified powder form is sprayed or brushed on the internal compound of the crude tire formed, then dried and vulcanized. Such a method necessitates perfect mastery of the drying phase and entails an operating time incompatible with an industrial application.
Another solution, such as patent WO 92/20538 describing the use of a reticular chlorinated polyethylene elastomer (CPE), whose oxygen permeability, weight and cost are reduced in relation to a butyl rubber inner liner, does not seem any more satisfactory, the airtightness function not being accomplished efficiently enough.
It is to be feared, moreover, that the thermoplastic character of most of the materials used is incompatible with the mechanical stresses to which the interior surfaces are subjected on different phases of assembly and vulcanization and on the rolling of said tires.